1. Technical Field
The present invention relates to an optical isolator which performs optical isolation with substantially no polarization mode dispersion.
2. Description of the Prior Art
Reflecting in optical systems often generate noise and optical feedback which may degrade the performance of various system components, particularly semiconductor lasers. Therefore, the ability to optically isolate lasers and other sensitive components from these reflections is critical to the performance of the system. The Faraday effect in magneto-optic material enables the construction of a unique non-reciprocal device capable of performing the isolation function.
To reduce the insertion loss for coupled fibers, an isolator should operate independent of the polarization state of the applied signal. In general, a conventional optical isolator comprises a 45.degree. Faraday rotator encased within a bias magnet and disposed between a pair of polarization selective means (e.g. birefringent plates, or birefringent wedges) oriented at an angle of 45.degree. to each other. The combination provides optical isolation in the reverse direction because the Faraday rotation means causes the two polarization states to switch identities as they pass through the birefringent devices.
One arrangement for eliminating polarization dependence is discussed in the article "Compact Optical Isolator For Fibers Using Birefringent Wedges", by M. Shirasaki et al., 21 Applied Optics 4296-99 (1982). In particular, Shirasaki et al. utilize a pair of birefringent wedges, located at the input and output of the Faraday rotator, to separate an incident beam into orthogonal, linear polarizations which travel independently through the isolator. Signals passing through the isolator in the forward transmitting direction will be essentially unaffected by the bireffingent wedges and Faraday rotator whereas, in the reverse direction, both polarization states undergo angular deviation so that neither polarization state is coupled to the input signal path of the isolator. Although the Shirasaki et al. arrangement, and other commercially available isolators, may be polarization independent, they can exhibit polarization mode dispersion in that the propagation time of a ray through the birefringent material is a function of its polarization state (i.e. extraordinary polarization state or ordinary polarization state). In particular, the birefringent material will have a different refractive index for each polarization state. As a result, a net dispersion (i.e. propagation delay between polarization states) that can be on the order of picoseconds will exist as the rays emerge from the isolator.
The net polarization mode dispersion At is calculated according to the formula: ##EQU1## where L is the total path length in the two wedges, c is the speed of light in free space, n.sub.o is the refractive index seen by the ordinary ray, and n.sub.e is the refractive index seen by the extraordinary ray.
For a standard Type 25 or Type 26 isolator manufactured by AT&T, which utilizes rutile material (n.sub.o =2.454 and n.sub.e =2.710), the net dispersion becomes: ##EQU2## For some applications, such as cascaded amplifiers in undersea systems, this degree of dispersion may present a serious problem. Accordingly, various techniques have been attempted in order to compensate for this dispersion through the inclusion of one or more additional compensation elements added to the isolator.
A need nevertheless remains in the art for a simple and effective optical isolation means that introduces substantially no polarization mode dispersion and which therefore requires no polarization mode dispersion correction.